net: mvpp2: update mvpp2 validate() implementation

Fix up the mvpp2 validate implementation to adopt the same behaviour as
mvneta:
- only allow the link modes that the specified PHY interface mode
  supports with the exception of 1000base-X and 2500base-X.
- use the basex helper to deal with SFP modules that can be switched
  between 1000base-X vs 2500base-X.

This gives consistent behaviour between mvneta and mvpp2.

This commit depends on "net: phylink: extend clause 45 PHY validation
workaround" so is not marked for backporting to stable kernels.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Acked-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>